EFFECT OF Z-PINS’ DIAMETER,SPACING AND OVERLAP LENGTH ON CONNECTING PERFORMANCE OF CMC SINGLE LAP JOINT

The effect of through-thickness reinforcement by composite pins (Z-pins) on the static tensile strength and failure mechanisms of the joints made from ceramic matrix composite (CMC) is investigated. Overlap length of the single lap joint is 15 mm, 20 mm, 23 mm, 37 mm, and 60 mm, respectively. The experimental results indicate that the final failure modes of the joints can be divided into two groups, (a) the bond-line stops debonding until crack encounters Z-pins; and then the adherends break at the location of Z-pins, when overlap length is more than 20 mm; (b) the bond-line detaches entirely and Z-pins are drawn from adherends, when overlap length is equal to 15 mm. A simple effcient computational approach is presented for analyzing the benefit of through-thickness pins for restricting failure in the single lap joints. Here, the mechanics problem is simplified by representing the effect of the pins by tractions acting on the fracture surfaces of the cracked bond-line. The tractions are prescribed as functions of the crack displacement, which are available in simple forms that summarize the complex deformations to a reasonable accuracy. The resulting model can be used to track the evolution of complete failure mechanisms, for example, bond-line initial delamination and ultimate failure associated with Z-pin pullout, ultimate failure of the adherends. The paper simulates connecting performance of the single lap joints with different Z-pins’ diameter, spacing and overlap length; the numerical results agree with the experimental results; the numerical results indicate enlarging diameter and decreasing spacing of Z-pins are in favor of improving the connecting performance of the joints. By numerical analysis method, the critical overlap length that lies between two final failure modes is between 18 mm and 19 mm, when Z-pins’ diameter and spacing are 0.4 mm, 5 mm, respectively.     

On reproducibility of preparation of basal plane pyrolytic graphite electrode surface

The basal plane pyrolytic graphite (BPPG) electrode is widely used as a “standard” electrode for studies of electrochemical activity of graphite, graphene and carbon nanotubes. We compared here different methods for preparation of the surface of BPPG using statistical analysis of the data. We showed that including/omitting of polishing step or including/omitting of acetone washing step in the preparation procedure exhibits statistically significant difference. However, using acetone of different purities does not lead into statistically significant results. In addition, all methods with BPPG provided RSD < 10% and did not produce any outlying results.     

Statistical properties of the energies and electric dipole moments of the bound vibrational states of the system HCO/COH

From the ab initio calculated three-dimensional adiabatic double-minimum potential energy surface of the HCO⁺/COH⁺ system and the corresponding dipole moment surface the energies of all bound vibrational states and their effective dipole moments are determined applying the Suttcliffe–Tennyson Hamiltonian for triatomic molecules. The energy and dipole data are analysed in terms of statistical methods such as the density of states approach and the nearest-neighbor level spacing distribution (NNSD). Special effort is put into investigating the effect of the tunnelling motion across the isomerization barrier on the NNSD representations.     

Level spacing statistics for two-dimensional massless Dirac billiards

Classical-quantum correspondence has been an intriguing issue ever since quantum theory was proposed. The searching for signatures of classically nonintegrable dynamics in quantum systems comprises the interesting field of quantum chaos. In this short review, we shall go over recent efforts of extending the understanding of quantum chaos to relativistic cases. We shall focus on the level spacing statistics for two-dimensional massless Dirac billiards, i.e., particles confined in a closed region. We shall discuss the works for both the particle described by the massless Dirac equation(or Weyl equation)and the quasiparticle from graphene. Although the equations are the same, the boundary conditions are typically different,rendering distinct level spacing statistics.     

Stacking faults blocking process in (1 1 −2 2) semipolar GaN growth on sapphire using asymmetric lateral epitaxy

We have succeeded in effectively stopping the propagation of basal stacking faults in (1 1 −2 2) semipolar GaN films on sapphire using an original epitaxial lateral overgrowth process. The growth conditions were chosen to enhance the growth rate along the [0 0 0 1] inclined direction. Thus, the crystal expands laterally until growth above the a-facet of the adjacent crystal seed, where the basal stacking faults emerge. The growth anisotropy was monitored using scanning electron microscopy. The faults filtering and improvement of crystalline quality were attested by transmission electron microscopy, X-ray diffraction and low temperature photoluminescence, which exhibits high intensity band-edge emission with low stacking faults related emission.     

The effects of annealing on non-polar (1 1 2¯ 0) a-plane GaN films

Non-polar a-plane (1 1 2¯ 0) GaN films were grown on r-plane sapphire by metal–organic vapor phase epitaxy and were subsequently annealed for 90 min at 1070 °C. Most dislocations were partial dislocations, which terminated basal plane stacking faults. Prior to annealing, these dislocations were randomly distributed. After annealing, these dislocations moved into arrays oriented along the [0 0 0 1] direction and aligned perpendicular to the film–substrate interface throughout their length, although the total dislocation density remained unchanged. These changes were accompanied by broadening of the symmetric X-ray diffraction 1 1 2¯ 0 ω-scan widths. The mechanism of movement was identified as dislocation glide, occurring due to highly anisotropic stresses (confirmed by X-ray diffraction lattice parameter measurements) and evidenced by macroscopic slip bands observed on the sample surface. There was also an increase in the density of unintentionally n-type doped electrically conductive inclined features present at the film–substrate interface (as observed in cross-section using scanning capacitance microscopy), suggesting out-diffusion of impurities from the substrate along with prismatic stacking faults. These data suggest that annealing processes performed close to film growth temperatures can affect both the microstructure and the electrical properties of non-polar GaN films.     

Raman and X‐ray diffraction analysis on unburned carbon powder refined from fly ash

The bonding and crystalline structures of oil‐fried fly ash collected from a power plant were analyzed by using Raman spectroscopy and X‐ray diffraction (XRD), respectively. These carbon powders underwent a series of annealing treatment for graphitization and crystallization. In Raman spectra, the refined, unburned carbon contains clearly the D, G and D′ peaks under 2000 or 2700 °C treatment. The Raman spectral line shape of refined, unburned carbon heated at 2700 °C is similar to that of commercial graphite made from graphitization process. In the XRD spectra, the refined, unburned carbon contains (002), (100) and (004) diffraction peaks under heating at 2000 or 2700 °C. The XRD spectral line shape of refined, unburned carbon heated at 2700 °C is similar to that of a commercial graphite bar. The quantitative graphitization level from Raman spectra and crystallization degree from XRD spectra on refined, unburned carbon powders serve as preliminary guide for the qualitative evaluation of these unburned carbon powders. Copyright © 2009 John Wiley & Sons, Ltd.     

Shear-band spacing controlled by Bridgman solidification in Dendrite/BMG composites

The paper reports that the shear-band spacing can be controlled by the Bridgman solidification technique for a model alloy of Zr38.3Ti32.9Nb7.3Cu6.2Be15.3.The volume percent of the glass phase is almost independent of the withdrawal velocity.The shear-band spacing reaches a minimum of about 3 μm with withdrawal velocity of 0.8 mm/s.The optimized mechanical properties,such as the fracture strength of ～2100 MPa and plastic strain of ～19%,have been obtained.The large plastic deformation is due to the dislocati...     

Fatigue striation spacing and equivalent initial flaw size in Al 2024-T3 riveted specimens

One of the main concerns of aeronautical structures is structural damage due to fatigue. This phenomenon is accentuated in areas of stress concentration, namely the connection of components. The fuselage is one of the most important structural elements where the connection of the fuselage panels is often done by riveting. Specimens with several geometric configurations are used to study these structures. This work is concerned with the study of one of these specimens, i.e., the lap splice with three rows of rivets and one rivet column. It is part of the IDMEC-Porto contribution to the European project ADMIRE. Stress versus number of cycles results and measurements of fatigue striation spacing along two perpendicular directions using scanning electron microscopy (SEM) are presented. The fractographic reconstitution of fatigue crack growth was carried out. The value of equivalent initial flaw size (EIFS) is calculated using the back extrapolation technique and two models of fatigue crack growth. They consist of a model using the Paris law and another the strain energy density factor concept. The EIFS is used to predict the fatigue behaviour of this structural detail. It is intended to verify the applicability of empirical models based in experimental evidences.     

Development of dendrite array growth during alternately changing solidification condition

To investigate competitive growth in a dendrite array a directional solidification study was carried out on a succinonitrile–acetone alloy. In the experiment the temperature gradient G applied is alternately altered over a wide range, increased from a lower to a higher limit, and then decreased back to the lower one. It was found that source dendrites, i.e. parent dendrites, are not superior to derived dendrites in terms of growth competition. The orientation deviation between neighbouring dendrites impacts both local dendrite creation and elimination. At lower gradients, the new dendrites grow out from the ternary arms, while at the higher gradients new dendrites originate directly from the secondary arms. During increasing and decreasing G, average primary dendrite spacing λ measured traces out different paths, revealing an unclosed hysteresis loop in the λ–G-diagram.